Polar animals' antifreeze has a spiky secret

2019-03-07 05:06:02

By Colin Barras TO SURVIVE in frigid polar regions, many cold-blooded creatures employ a natural antifreeze to protect themselves from the damage that large ice crystals would cause. These antifreeze molecules lock onto ice crystals, but not liquid water – though how they do this has been a mystery. Now the mechanism has been revealed, opening the way to using similar molecules in cancer treatments, to protect healthy tissue while tumours are destroyed by freezing. Antifreeze proteins (AFPs) found in nature lock onto ice crystals and stop them growing large enough to damage tissue. If AFPs bound as easily to liquid water as they do to ice, this lifesaving action could turn killer, as animals would quickly dehydrate, says Matthew Blakeley at the Laue-Langevin Institute in Grenoble, France. X-ray diffraction has already helped to show that the surface of the AFPs that binds to ice is covered in tiny hydrophobic spikes. Now, using neutron diffraction, which is more effective at showing up the hydrogen in water molecules, Blakeley and colleagues managed to catch four water molecules at the ice-binding surface of AFPs from a fish, the ocean pout (Zoarces americanus). The water molecules formed an arc resembling part of a six-molecule ring characteristic of ice crystals. At the centre of each ring is a nanoscale hole. Crucially, the hole in the centre of the partial ring was occupied by one of the hydrophobic spikes. The distance between spikes suggests that each would fall inside one of the holes if an ice crystal were present (Journal of Molecular Recognition, DOI: 10.1002/jmr.1130). While this formidable array of hydrophobic spikes deflects liquid water molecules from the protein, says Blakeley, their alignment with the ice holes means they fit neatly into ice. “It’s almost like a lock and key.” The formidable array of spikes deflects liquid water molecules, but locks onto ice crystals Confirmation of this picture comes from Peter Davies at Queen’s University in Kingston, Ontario, Canada, and his team. Davies says they “got lucky” and glimpsed 60 water molecules – not just four – on an AFP using X-ray diffraction. Again, the spikes slotted into holes in the water’s structure (Proceedings of the National Academy of Sciences, DOI: 10.1073/pnas.1100429108). The insight could lead to the use of AFPs in “cryosurgery” to destroy tumours. More on these topics: